"Our hope was that by embedding spider-silk protein [gene] sequences within silkworm silk [gene] sequences, we could get those proteins to co-assemble ... into composite fibers, and that is what happened," said study co-author Don Jarvis, a molecular biologist at the University of Wyoming in Laramie.

The genetically engineered silkworms produced silk that was still 96 to 98 percent silkworm, with only 2 to 4 percent of the fiber proteins coming from spiders, Jarvis said.

Even with such a small uptake of spider genes, the hybrid silk was more than twice as tough as natural silkworm silk—yet still about half as strong as spider silk.

"If we can drive this [effect] from 2 to 4 percent to a hundred percent, we should be able to produce truly amazing fibers," Jarvis said.

"It just reflects how much of the spider silk is being produced in the soluble form."